Application of flat panel digital imaging for improvement of ocular melanoma patient set-up in proton beam therapy

In order to reduce the dose to surrounding critical tissues and also minimize the probability of recurrence of the tumor the placement of radiation fields relative to patient anatomy is very essential in proton beam therapy of ocular tumors. To achieve this objective, patient setup and field placement have been verified before treatment by analyzing the portal images obtained with Polaroid film-camera system. The Polaroid films are becoming expensive and obsolete, making new methods of verifying the patient treatment position essential. jective of this study was to implement an orthogonal flat panel digital imaging (FPDI) system as a tool to image-guided radiation therapy (IGRT) on the UC Davis cyclotron proton beam therapy line and to use the system for patient setup verification. The image quality of the system is sufficient to see an air hole with a diameter of 0.5 mm at a depth of 9 mm, in a 10 cm Lucite phantom. The subject contrast of the FPDI system varied from 16% to 29% by varying the size of the air hole in the phantom from 1 to 5 mm and changing the depth from 9 to 15 mm. The subject contrast for 0.5 mm air hole was 11%. mparison of the setup variations as measured from Polaroid port films and FPDI was 0.1±0.7 mm in the X-direction, 0.2±0.2 mm in the Y-direction and 0.04±0.1 mm in Z-direction, respectively. The day-to-day positional variations in-patient set-ups were studied for 30 patients using the FPDI system. The patient position set-up on first day of treatment [defined by the X, Y, Z coordinates of the chair and head holder] was registered as the reference image. The comparison of day-to-day patient position with reference image indicated net translation along the three orthogonal axes as 0.3±1.88 mm in right–left direction, −0.3±1.78 in superior–inferior direction and −0.6±2.8 mm in anterior–posterior direction. The image quality of the FPDI system was sufficient to clearly reveal the radio-opaque markers on the digital image. clusion a FPDI system can accurately replace the Polaroid system and will facilitate daily portal alignment and true electronic IGRT verification of patient position and tumor location relative to the proton beam.